A comprehensive study led by Peter J. Flood, a postdoctoral research fellow at the University of Michigan’s School for Environment and Sustainability (SEAS), has revealed alarming trends regarding fish sizes in Michigan’s inland lakes. The national study, which spanned an impressive 75-year period of data collection, examined over 1,500 lakes. Their results show that fish populations—adults as well as juveniles—are dramatically declining due to climate change.
The collaborative study involved a team of researchers including Katelyn King, Kevin Wehrly, and senior author Karen Alofs, an associate professor at SEAS. They analyzed 125 species-age class combinations and found that in 58 of these combinations, a change in size had already occurred. Most impressively, 46 of these combinations had a statistically significant trend toward smaller size.
One of the most alarming aspects of the study is that the most significant reductions in fish length happened to the youngest and oldest fish. This unexpected result brings to question the evolutionary ecology equilibrium existing within these aquatic landscapes. Largemouth bass, a species better suited to warmer climates, are now more common as Michigan’s lakes have started to warm.
The team did tremendous background work by using a community science project. Each of them worked to digitize decades’ worth of observation cards, that detailed the nuances of fish populations over time in 1,497 inland lakes across the state of Michigan. The University of Michigan’s Division of Fishes houses approximately 3.5 million specimens from various locations worldwide, serving as a vital resource for understanding the impacts of climate change on fish populations.
Flood remarked on the implications of these findings: “Climate change is altering the size of different organisms around the world, including fishes in lakes here in Michigan.” He emphasized that “most of those changes we’re seeing in Michigan fishes are declines in size through time.”
The study illuminated a troubling trend: mass mortality events among fish often occur during the spring ice thaw. Now, as a consequence of reduced ice cover, these events are pushing later into the year. This recent change may tremendously impact survival and population dynamics of fishes. Flood stated, “Each of these changes, whether size, abundance or mortality, has important implications for these ecosystems.”
Future studies will encompass museum samples such as lake trout. This will allow us to examine the impact of climate change on fish assemblages in Michigan’s inland lakes. Flood expressed optimism regarding the data set, stating, “There’s still so much more that can still be done with this data set.” He added that the study serves as a valuable tool for fisheries managers: “Our study is showing that there are differential responses of certain age ranges, so this is a tool in the toolbox managers can use to try to mitigate some of these climate change effects.”